The invention relates to a method and an apparatus for inductively or capacitively measuring shifts of the lateral position of a longitudinally moving metal strip.
EP 0 529 440 discloses, for the purpose of inductively measuring and controlling the position of a moving metal strip, providing, on one side of said metal strip, e.g. a primary coil fed by an AC voltage below the strip edge and, opposite said primary coil, a secondary coil above the strip edge, an exciter circuit being provided for feeding the AC voltage into the primary coil and a detector circuit for determining the voltage induced in the secondary coil as a result of the coupling effect with the primary coil. The stationary primary and secondary coils have the shape of an approximately elongate rectangle, and they are arranged with their longitudinal axis transverse with respect to the strip edge. The measurement signals determined by the coils are processed for the strip travel control, in order to keep the strip in a predetermined position. This structure corresponds to that shown in FIG. 1.
In order to compensate for the influence of different material properties and/or different thickness dimensions of the strip during the measurement, it is also known to provide a reference sensor in the form of a primary and secondary reference coil in the center of the strip between the primary and secondary coils on both strip edges. This reference sensor ascertains the minimum possible coupling, the signal obtained by the reference sensor being subjected to further processing with the signals obtained by the two strip edge sensors.
The arrangement of a reference sensor between the primary and secondary coils requires a certain amount of space, and different strip widths, which likewise influence the measurement signal, cannot be compensated for by such a reference sensor. In addition, a known configuration of this type is not suitable for narrow strip widths, for example if the strip width is of the order of magnitude of the dimensions of the reference sensor.
The invention is based on designing a method and an apparatus of the type specified in the introduction in such a way that, with the smallest possible space requirement, sidewise variations of position of different moving strip materials can be measured exactly and, in particular, it is also possible to compensate for any change in the measurement signal resulting from the use of different strip widths.
By virtue of the fact that spaced-apart coils are arranged at opposite edges of the moving metallic strip and, as a result of a relative movement between the coils and the deflection of the strip transversely with respect to the direction of strip movement, the measurement gradient is determined and compared to the optimum measurement gradient electronically for the strip movement control, it is possible, irrespective of the different material properties of the strip and/or its thickness and width dimensions, and/or the throughput level of the strip, to obtain a measurement signal which represents the maximum voltage change as a function of the sidewise deflection of the strip, with the result that optimum control conditions are always obtained.
Compensation of different measurement signals is thus achieved without a reference sensor, as a result of which the measuring apparatus can be designed to be very compact and also effective for narrow strip widths.